A two-stroke cycle engine is an internal combustion engine that completes a power cycle with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine in which two pistons are disposed in opposition in the bore of a cylinder for reciprocating movement in opposing directions. The cylinder has longitudinally-spaced inlet and exhaust ports that are located near respective ends of the cylinder. Each of the opposed pistons controls one of the ports, opening the port as it moves to a bottom center (BC) location, and closing the port as it moves from BC toward a top center (TC) location. One of the ports provides passage for the products of combustion out of the bore, the other serves to admit charge air into the bore; these are respectively termed the “exhaust” and “intake” ports. In a uniflow-scavenged opposed-piston engine, charge air enters a cylinder through its intake port and exhaust gas flows out of its exhaust port, thus gas flows through the cylinder in a single direction (“uniflow”)—from intake port to exhaust port.
In FIG. 1, a uniflow-scavenged, two-stroke cycle internal combustion engine is embodied by an opposed-piston engine 49 having at least one ported cylinder 50. For example, the engine may have one ported cylinder, two ported cylinders, three ported cylinders, or four or more ported cylinders. Each ported cylinder 50 has a bore 52 and longitudinally-spaced exhaust and intake ports 54 and 56 formed or machined in respective ends of a cylinder wall. Each of the exhaust and intake ports 54 and 56 includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid bridge. In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions shown in FIG. 1. In the example shown, the engine 49 further includes two crankshafts 71 and 72. The exhaust and intake pistons 60 and 62 are slidably disposed in the bore 52 with their end surfaces 61 and 63 opposing one another. The exhaust pistons 60 are coupled to the crankshaft 71, and the intake pistons are coupled to the crankshaft 72.
As the pistons 60 and 62 near TC, a combustion chamber is defined in the bore 52 between the end surfaces 61 and 63 of the pistons. Fuel is injected directly into the combustion chamber through at least one fuel injector nozzle 100 positioned in an opening through the sidewall of a cylinder 50. The fuel mixes with charge air admitted into the bore through the intake port 56. As the air-fuel mixture is compressed between the end surfaces it reaches a temperature that causes combustion.
With further reference to FIG. 1, the engine 49 includes an air handling system 51 that manages the transport of charge air provided to, and exhaust gas produced by, the engine 49. A representative air handling system construction includes a charge air subsystem and an exhaust subsystem. In the air handling system 51, the charge air subsystem includes a charge source that receives fresh air and processes it into charge air, a charge air channel coupled to the charge air source through which charge air is transported to the at least one intake port of the engine, and at least one air cooler in the charge air channel that is coupled to receive and cool the charge air (or a mixture of gasses including charge air) before delivery to the intake port or ports of the engine. Such a cooler can comprise an air-to-liquid and/or an air-to-air device, or another cooling device. The exhaust subsystem includes an exhaust channel that transports exhaust products from exhaust ports of the engine for delivery to other exhaust components.
With further reference to FIG. 1, the air handling system 51 includes a turbocharger 120 with a turbine 121 and a compressor 122 that rotate on a common shaft 123. The turbine 121 is coupled to the exhaust subsystem and the compressor 122 is coupled to the charge air subsystem. The turbocharger 120 extracts energy from exhaust gas that exits the exhaust ports 54 and flows into an exhaust channel 124 directly from the exhaust ports 54, or from an exhaust manifold 125 that collects exhaust gasses output through the exhaust ports 54. In this regard, the turbine 121 is rotated by exhaust gas passing through it. This rotates the compressor 122, causing it to generate charge air by compressing fresh air. The charge air subsystem includes a supercharger 110. The charge air output by the compressor 122 flows through a charge air channel 126 to a cooler 127, whence it is pumped by the supercharger 110 to the intake ports. Charge air compressed by the supercharger 110 can be output through a cooler 129 to an intake manifold 130. The intake ports 56 receive charge air pumped by the supercharger 110, through the intake manifold 130. Preferably, in multi-cylinder opposed-piston engines, the intake manifold 130 is constituted of an intake plenum that communicates with the intake ports 56 of all cylinders 50.
In some aspects, the air handling system shown in FIG. 1 can be constructed to reduce NOx emissions produced by combustion by recirculating exhaust gas through the ported cylinders of the engine. The recirculated exhaust gas is mixed with charge air to lower peak combustion temperatures, which reduces production of NOx. This process is referred to as exhaust gas recirculation (“EGR”). The EGR construction shown obtains a portion of the exhaust gasses flowing from the port 54 during scavenging and transports them via an EGR loop external to the cylinder into the incoming stream of fresh intake air in the charge air subsystem. Preferably, the EGR loop includes an EGR channel 131. The recirculated exhaust gas flows through the EGR channel 131 under the control of a valve 138 (this valve may also be referred to as the “EGR valve”).
In many two-stroke engines, combustion and EGR operation are monitored and optimized based on various measurements related to the amount of charge air delivered to the engine. For example, the ratio of the mass of charge air delivered to a cylinder to the reference mass of charge air required for stoichiometric combustion in the cylinder (“lambda”) is used to control NOX emissions over a range of engine operating conditions. However, in a two-stroke cycle opposed-piston engine with uniflow scavenging, port opening times overlap for a portion of each cycle and some of the charge air delivered to a cylinder through its intake port flows out of the cylinder before the exhaust port is closed. The charge air flowing out of the exhaust port during scavenging is not available for combustion. Thus, a value of lambda based on charge air delivered (“delivered lambda”) to the intake port of a cylinder in an opposed-piston engine with uniflow scavenging overstates the amount of charge air actually available for combustion.
Accordingly, there is a need to improve the accuracy of air handling control in uniflow-scavenged, opposed-piston engines.